More precisely, the invention concerns a process for the manufacture of a substrate that includes a top layer in a first material and an underlying layer in a second material whose lattice parameter is different from the lattice parameter of the first material.
In one example of implementation of the invention, the first material is relaxed SiGe or Ge. In this example, the second material is Si, whose lattice parameter is different from the lattice parameter of the layer of first material (SiGe or Ge).
Still in this example, the top relaxed SiGe or Ge layer is generally separated from the layer of Si by a transition layer in which the lattice parameter changes in a continuous manner between these two layers. A transition layer in SiGe, in which the concentration of Ge and therefore the lattice parameter changes according to a gradient when one passes through the layer in the direction of its thickness is an example of such a transition layer. The concentration of Ge can reach a value that is typically between 20% to 100%.
It should be noted that these examples of first and second materials do not limit the invention.
Returning however to the type of substrate mentioned above, the invention thus applies in a particularly advantageous manner to the manufacture of a substrate that comprises a top layer in relaxed SiGe (or Ge) and an underlying layer in SiGe which thus plays the role of a transition layer between the top layer and a support layer (in Si, for example).
Such a substrate is represented in FIG. 1.
This substrate (10) includes a support layer (100) in Si, a top layer (120) in SiGe (SiGe with 20% of Ge, for example,—which will be referred to as SiGe 20%), and a transition layer (110) in which the concentration of Ge increases from 0% (at the contact with the support layer) to 20% (at the contact with the top layer).
As we will see, the invention also concerns a substrate that is obtained by a process such as that mentioned above. Such a substrate according to the invention can in particular be of the general type shown in FIG. 1 (where the proportions of Ge can be different from those that have just been mentioned concerning the example in this figure).
Processes such as those mentioned above are already known.
It is thus possible to obtain substrates such as that represented in FIG. 1, for example.
We also know about then covering the top layer, in which the concentration in Ge is constant, with a layer of strained Si (sSi).
However, the known processes are associated with some limitations.
A first limitation concerns the presence of defects of the dislocation type and their agglomerates (commonly called “pile-ups”,) in the top layer. It is thus difficult to obtain layers that have dislocation densities of less than 1E5-1E6 #/cm2 and “pile-up” densities of less than 5-20 cm /cm2.
These defects affect the crystalline structure of the top layer, and alter its quality. The dislocations are also liable to propagate through the whole thickness of the layer.
A second limitation concerns the surface state of the top layer.
In fact at the surface of the layers such as layer 120 in FIG. 1, one sees a topology of average roughness (“peak to valley” factor), which results from stresses in the thickness of the top layer.
This surface state is commonly referred to as “crosshatching”.
This crosshatching can be characterised by Atomic Force Microscopy, in the case of a relaxed SiGe layer 20% (meaning one with 20% of Ge), by a roughness of the order of 30 angstroms RMS and 200 angstroms (“Peak to Valley” or PV roughness) for scanned surfaces of 40*40 microns. FIG. 2 illustrates such a surface state.
Thus, the known processes are associated in particular with two drawbacks, namely defects of the dislocation type, and crosshatching.
Techniques that aim to avoid at least one of these drawbacks are known. These techniques will be described below, here again returning to the example of a substrate whose top layer is in SiGe.
A first technique consists in treating the surface of the substrate by chemical-mechanical polishing CMP, and then causing again SiGe to grow on the polished surface.
According to this technique, the creation of the substrate is interrupted during the growth of the relaxed SiGe layer in order to execute a stage of rectification by CMP of the surface state of the SiGe layer that has already been constituted, thus reducing the roughness. After the CMP stage, the growth of the relaxed SiGe layer resumes.
It would appear that this roughness elimination stage results, during later growth, in favouring the slippage and the disappearance of the dislocations within the relaxed SiGe layer.
A second technique consists in subjecting the substrate to an annealing process, and then to an CMP stage, once the said substrate has been constituted.
The annealing is conducted at high temperature (more than 900° C., for 2 hours).
This can favour stabilisation of the substrate and dissipation of the residual stress in the relaxed SiGe layer.
The later CMP treatment is then applied to rectify the surface state of the layer.
A third technique consists in increasing the thickness of the transition layer that is positioned between the top SiGe layer and the support layer.
This is used to change the lattice parameter of this transition layer in a very progressive manner—typically by adopting a Ge concentration gradient that is lower than the one that would be adopted in the absence of dislocation risk.
It is possible that the techniques described above might provide a solution to the drawbacks mentioned earlier.
However, the implementation of these techniques involves stages that render the process complex and/or costly.
It is also said, concerning the advantages to be gained from using CMP, that it as been observed that even after planarisation (that is flattening or smoothing) of a surface by CMP, crosshatching could re-appear during later stages of the treatment to which the substrate was subjected, after transfer or after a simple thermal treatment for example.
This suggests the crosshatching is the expression of a complex phenomenon at the crystalline level and occurring in the thickness of the layer, a phenomenon which was not treated by the techniques described above.
It therefore appears that the known processes are associated with some limitations.
The aim of the invention is to enable these limitations to be overcome.